The present invention relates to ion conductive matrixes, membranes and electrodes, their manufacture and use. In particular, the present invention is concerned with membranes comprising composite polymeric films and composite polymers.
Ion conducting membranes (to be referred to hereinafter as xe2x80x9cICMxe2x80x9d) are to be found in many electrochemical cells, among which: fuel cells, electrolyzers, electrochromic cells, batteries, electrochemical sensors and others. In some cases, polymer electrolyte is used such as Nafion. However, Nafion based fuel cells suffer from two major disadvantages. The first is that Nafion is a very expensive material and the second, its characteristic to dry out during the fuel cell operation due to water dragging by the protons conducted.
U.S. Pat. No. 5,456,600 teaches the use of a polymeric membrane for making an lithium-ion rechargable battery cell. The membrane disclosed is a combination of a poly(vinylidene fluoride) copolymer matrix and a compatible organic solvent plasticizer which maintains a homogenous composition in the form of a flexible, self-supporting film.
U.S. Pat. No. 5,643,689 discloses a non-liquid proton conductor membrane which comprises a matrix polymer dissolvable in a solvent and an acidic multimer dissolvable in that solvent. In accordance with the disclosure of this publication, when the membrane is contacted with a second solvent, it swells to allow an improved electrical contact between the cathode and anode plates in which the membrane is interposed.
Another type of polymer membrane is described in U.S. Pat. No. 5,425,865. The membranes described comprise a porous matrix of a crosslinked polymeric material, and a second polymeric material that partially blocks the pores of that matrix.
Another type of cell is the phosphoric acid fuel cell (PAFC), wherein acid is absorbed in a porous matrix made of e.g. silicon carbide powder and metal phosphate or metal oxide or metal salt. Typically, such a cell operates at elevated temperatures, about 180xc2x0 to 200xc2x0 C. U.S. Pat. No. 4,623,415 describes a membrane comprising a porous matrix for retaining phosphoric acid electrolyte where the matrix consists of a substance which is unreactive with phosphoric acid and has electron insulating properties.
It is an object of the present invention to provide a novel, low cost and highly conductive ion conducting matrix, i.e. a matrix in which ions derived from acids, bases or salts are mobile.
It is another object of the present invention to provide novel, low cost and highly conductive ion conducting membranes and electrodes.
Another object of the present invention is to provide electrochemical cells comprising the matrixes of the invention.
It is yet a further object of the invention to provide methods for preparing such membranes and electrodes.
Still, it is the object of the present invention to provide uses for the ion conducting membranes and electrodes invention.
Other objects of the invention will become apparent as the description of the invention proceeds.
Thus, the present invention provides by the first of its aspects an ion conducting matrix comprising:
(i) 5% to 60% by volume of an inorganic powder having a good aqueous electrolyte absorption capacity;
(ii) 5% to 50% by volume of a polymeric binder that is chemically compatible with an aqueous electrolyte; and
(iii) 10 to 90% by volume of an aqueous electrolyte,
wherein the inorganic powder comprises essentially sub-micron particles, preferably from about 5 to about 150 nm in size. The matrix of the present invention may optionally comprise between about 0.1% to about 25% of a non-volatile liquid lubricant that is chemically compatible with all the components in the matrix.
In accordance with a preferred embodiment of the present invention, the inorganic powder is characterized in that it has a surface area of at least 10 m2/g, and possesses a good absorption capability for the aqueous electrolyte.
According to another aspect of the invention, there is a provided a membrane being a film made of the matrix of the invention.
According to a further aspect of the invention, there is provided a composite electrode comprising 10 to 70% by volume of the matrix of the invention and the balance is made essentially by an electrode material which is a material known in the art per se as a suitable material in the manufacturing of electrodes, eg. carbon, graphite, air, oxygen, H2, methanol electrodes, Zn, Cd, Ni, Pb, Fe, Cu or their alloys, metal oxide electrodes, e.g. RuO2, WOx, MnO2, NiOOH, AgO, Ag2O and the like.
In the case that the matrix of the invention is used as an ion conducting matrix in a composite electrode, the inorganic powder may be electronically conductive.
Preferably, the inorganic powder of the matrix of the present invention is a member selected from the group consisting of SiO2, ZrO2, B2O3, TiO2, Al2O3 and the like.
The polymeric binder used in the matrix of the present invention is a material which is chemically compatible with an aqueous electrolyte used, i.e. non-soluble in that electrolyte, and is a member selected from the group consisiting of: polyvinilyden fluoride (PVDF), PVDF-hexafluoropropylene (PVDHFP), poly(tetrafluoroethylene) (PTFE), poly(methylmethacrylate) (PMMA), polysulfone amide, poly(acrylamide), polyvinyl chloride (PVC), poly(acrylonitrile), polyvinyl flouride and any combination thereof.
The aqueous electrolyte of the present invention consists of an aqueous soluble compound selected from a salt, a base or mixtures thereof. Examples of aqueous soluble salts are alkali metal salts, alkali earth metal salt, R4NX where R is hydrogen or an organic radical and X is an anion derived from an inorganic acid, NH4Cl, ZnCl2 and any combinations thereof.
Examples of aqueous soluble bases for use in the present invention are R4NOH where R is hydrogen or an organic radical alkali or alkali earth base compounds and any combinations thereof.
In the case that the matrix of the invention is used as a proton conducting matrix it is referred to hereinafter as xe2x80x9cTCMxe2x80x9d. The acid according to the present invention, which may be also a mixture of acids, may be a pure acid or an acid dissolved in water or in another suitable non-aqueous solvent that is known per se in the art. Acids suitable according to the present invention are: CF3(CF2)nSO3H, HO3S(CF2)nSO3H wherein n is an integer having a value of 0 to 9, sulfuric acid, HCl, HBr, phosphoric acid, HNO3 and the like. Preferred acids are CF3(CF2)nSO3H or HO3S3S (CF2)nSO3H where n is equal to 0, 1, 2, 3 or 4. These preferred acids can be used either in their pure form or as aqueous solutions having a molar concentration of 10 to 99%, and preferably a molar concentration of 25% to 99%.
The ICM of the present invention has the general appearance of a plastic film having good mechanical properties. It can typically be bent to about 180xc2x0 with no substantial fractures occurring, and it can be prepared in thickness being in the range of from about 10 to about 1000 microns or more. Due to its stability and good ionic conductivity, it can be used at a large temperature range of from sub-zero to about 150xc2x0 C.
According to a preferred embodiment of the invention, where the matrix is in the preparation of a membrane, the inorganic powder comprised in the matrix is a very fine, electronically non-conductive powder having a particle size of preferably less than 150 nm. According to this embodiment, the ICM pores in which the aqueous electrolyte is absorbed are very small, and their characteristic dimension is essentially smaller than 50 nm.
The absorption capacity or the retention capability of the membrane for the acid or the aqueous electrolyte used depends on several parameters, among which are the composition and the type of the inorganic powder, the polymeric binder and the type of the dissolved acid or electrolyte. The combination of these parameters should be optimized in order to tailor the product for each application. While carrying out such optimization, consideration should be given to the fact that the highest the content of inorganic powder is, the inferior the mechanical properties become. Increasing the inorganic powder content of the matrix increases its electrolyte retention characteristic, but at the same time, decreases its mechanical strength. On the other hand, increasing the polymeric binder in the matrix increases the strength of the latter, but decreases the wettability of the matrix thus turning it to a less conductive one.
According to yet another embodiment of the invention, an improvement of the matrix wettability and consequently the electrolyte retention, is achieved by adding to the membrane multi valance metal salts such as Al, Zr, B, Ti and the like.
According to another embodiment of the invention, the improvement of the matrix wettability and consequently the electrolyte retention is achieved by pre-treating the inorganic powder with an acid or a base prior to the preparation of the membrane.
The elasticity and elongation of ICMs that are rich in inorganic powder and that of electrodes based on these materials were improved by the addition of low to medium molecular weight (typically 100 to 7500 mass units) oil or oil like liquid materials that serve as an internal lubricant between the molecules of the polymer binder. The main effects of the intend lubricant are to increase the distance between the polymer chains, thus reducing the attraction forces between the polymer molecules and/or replacing the original attraction forces between the polymer molecules with lower attraction forces between the internal lubricant molecules and/or becoming a separation layer between the polymer molecules. ICMs that contain an internal lubricant have smaller resistance to external forces because the polymer chains can slide more easily one over the other. It is essential that these materials have low vapor pressure and are compatible with all other components at the relevant temperatures of application. In order to reduce the escape effects of an internal lubricant from the ICM (lower migration rates of these materials to the surface of the polymer) it is recommended, according to the present invention, to use an internal lubricant with a low vapor pressure, to use liquid polymeric internal lubricants or to chemically link the internal lubricant to the polymer molecules. Some of these lubricants may be useful only at room temperature and at mild conditions (e.g. not in strong acids or bases).
It was found, according to the present invention, that during the preparation process one of the following factors must be controlled:
a) The solubility parameter (xcex4-factor) of the internal lubricant should not differ by more than 5 (cal/cc)1/2 units from that of the polymer.
b) The solubility parameter (xcex4-factor) of the mixture of an internal lubricant or lubricants (in case of a combination of lubricants) and the solvent or solvents (in case of a mixture of solvents) will not differ by more than 5 units from that of the polymer of the matrix.
(Solubility parameters are defined in: xe2x80x9cPolymer Handbookxe2x80x9d, Interscience Publishers, a division of John Wiley and Sons, 1966, pp. IV-344-IV-367; J. E. Mark, Physical Properties of Polymers Handbook, American Institute of Physics, Woodbury, N.Y., 1996. pp. 231-237 and R. C. Weast CRC Hand Book of Chemistry and Physics, 56th Edition, 1975-1976, published by CRC Press.)
There is an advantage in using internal lubricants that have a low solubility in water so as to achieve a solubility factor which is not higher than 14 (cal/cc)1/2, preferably not higher than 10 (cal/cc)1/2. The low solubility prevents the migration of internal lubricant out of the ICMs when they come in contact with water at the washing phase or at the acid loading phase.
The internal liquid lubricant materials are selected, for example, from the following materials: diesters of aliphatic and aromatic dibasic acids such as, for example, adipic acid, phthalic acid and sebacic acid, esters of phosphoric acid, hydrocarbons and synthetic hydrocarbons such as decan, dodecan, etc., machine oil, silicone oils, fluorocarbons such as perfluoropolyethers, for example: CF3O[xe2x80x94CF(CF3)xe2x80x94CF2Oxe2x80x94]x(CF2Oxe2x80x94)yCF3, X being between 0 and 50 and y between 0 and 100. It was found that the addition of these lubricants increases in some cases the conductivity of the ICM by over 30%.
Still by another embodiment of the present invention, there is provided a membrane as described above that is mechanically reinforced. The reinforcement can be done by any way known per se in the art, e.g., including in the membrane an electronically non-conductive screen, felt, fibers or any other reinforcing element as known in the art.
The ion conducting membranes of the invention may be prepared by any one of several methods, which are also encompassed by the present invention, among which are casting and extrusion. The method for casting an ICM according to the present invention comprises the following steps:
(i) preparing a mixture comprising an inorganic powder, a polymeric binder that is chemically compatible with an aqueous electrolyte, at least one solvent characterized in having a high boiling point of above 100xc2x0 C. and at least one low boiling point solvent, having a boiling point lower than that of the high boiling point solvent(s), in which the polymeric binder is soluble or form a gel at the casting temperature;
(ii) casting a film out of the mixture;
(iii) allowing the low boiling point solvent to evaporate from the mixture, thus forming a solid film;
(iv) washing the solid film and replacing the high boiling point solvent with the desired aqueous electrolyte solution to be included in the membrane.
According to a preferred embodiment of the invention, the mixture is prepared in a paste-like gel or gel form and is introduced into a mold to obtain the required film form. The evaporation of the low boiling solvent as described in step (iii) should not necessarily be completed prior to proceeding to step (iv), and it may suffice that a solid film which can be further processed, is obtained. The film is then washed, preferably first with water and then with the electrolyte to be absorbed in the matrix, forming the required membrane. This step is carried preferably at a temperature of less than 150xc2x0 C. The use of water or the electrolyte may be done by repeating immersions of the film therein so as to displace the water or the high boiling solvent, as applicable. Preferably, the last immersion is conducted at an elevated temperature so as to evaporate the solvent and allow the aqueous electrolyte solution to replace it in the membrane formed. When using a water non-soluble high boiling point solvent the process involves washing the solid film with another solvent that is water soluble, followed by washing the solid film with water.
When casing a composite electrode which comprises the matrix of the present invention, the mixture prepared in step (i) of the above-described method, comprises a further powder of the suitable electrode material. The remaining steps of the process are carried mutatis mutandis as described in the method for casting an ICM.
According to a further embodiment of the invention, the high boiling point solvent is a water soluble solvent, and is preferably a member selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), dimethyl phthalate, diethyl phthalate, dibutyl phthalate and the like of any combinations thereof.
According to another embodiment of the invention, the low boiling point solvent is a member selected from the group consisting of tetrahydrofuran (THF), DME, cyclopentanon, acetone, N-methyl pyrrolidon (NMP), dimethylacetamide (DMAC), methylethylketone (MEK), dimethylformamide and the like, or any combination thereof.
The method for extruding an ICM according to the invention comprises the following steps:
(i) preparing a mixture comprising an inorganic powder, a polymeric binder that is chemically compatible with an aqueous electrolyte and at least one solvent characterized in having a boiling point of above 90xc2x0 C. in which the polymeric binder is at least partially soluble or form a gel at extrusion temperature;
(ii) heating the mixture to its softening temperature;
(iii) forming a film out of the mixture by hot extruding the softened mixture;
(iv) cooling the film thus formed;
(v) washing the solid film and replacing the solvent with the desired aqueous electrolyte solution to be included in the membrane.
When extruding a composite electrode which comprises the matrix of the present invention, the mixture prepared in step (i) of the above-described method comprises a further powder of the suitable electrode material. The remaining steps of the process are carried mutatis mutandis as described in the method of extruding an ICM.
According to a preferred embodiment of the invention, the solvent suitable for the extrusion method is a water soluble solvent being a member selected from the group consisting of propylene carbonate, diethyl carbonate, dimethyl carbonate butryoalactone, methyl isoamyl ketone, cyclohexanone, dialkyl phthalate, glycerol triacetate and the like or any combinations thereof.
The washing of the film described in step (v) is preferably done first with water and then with the aqueous electrolyte to be dissolved into the matrix, to form the required membrane. Water or the electrolyte is applied by repeatedly immersing the film therein so as to displace the water or the solvent, as applicable.
Another aspect of the invention concerns possible uses of the matrixes of the invention. One such use is the manufacture of electrochemical cells that are substantially free from mobile liquid electrolyte solution. Electrochemical cells may comprise an ICM sandwiched by two electrodes. Such electrodes are carbon or graphite, Zn, Fe, Cd, Ni, Cu, Al and their alloys; electrodes or metal oxide electrodes, e.g. of RuO2, WOXx, MnO2, NiOOH, AgO, Ag2O and the like. This type of a cell may be manufactured by using a hot press technique. In this process, the two electrodes and the ICM therebetween are pressed together at the softening temperature of the binder, with or without a solvent, typically at a temperature in the range of from about 60xc2x0 to 150xc2x0 and under 20 to 1000 Kg/cm2 for about 1 to 10 minutes, obtaining a single structural cell unit with improved mechanical strength and performance.
According to a preferred embodiment of the invention, a Zn cell may also be manufactured by hot pressing together a cathode, an anode and an ICM therebetween. In this process the anode consists of a ceramic powder binder such as PVDF and Zn in the form of a fine powder; the cathode may also consist of PVDF, ceramic powder, a metal oxide such as MnO2 and a small amount of carbon or graphite. In case that a Zn air cell is assembled, the cathode is a commercial air electrode made of platinum or another catalyst supported on carbon or graphite particles, paper or felt.
Cells such as Zn/air or Zn/oxygen cells consisist of 1) a composite Zn electrode made typically of about 30% v/v the ICM matrix and 70% of Zn in the form of a fine powder. The preferred Zn powder consists of up to 0.1% (w/w) Hg, Sn, In, Bi, Pb or mixture thereof for corrosion prevention; 2) an ICM; 3) an oxygen or air electrode. The air or oxygen electrode consists of a catalyst selected from Pt, Pd, W, Mn, Cu, Ag, Ni or their mixture or their oxides supported by carbon or graphite powder. The amount of the catalyst is from about 5% to about 50% w/w relative to the carbon or graphite powder.
One side of the air or oxygen electrode is hydrophilic and the other side is hydrophobic. Commercially available air or oxygen electrodes can be used or they can be made with the ICM matrix where the hydrophobic side contains more than 25% v/v polymer, preferably Teflon and no ceramic powder.
For example, according to the present invention, nickel-cadmium and nickel-iron batteries consist of a cathode made of NiOOH powder and the ICM matrix, while the anode is made of either cadmium based powder and ICM matrix or iron based powder and the ICM matrix.
In all cases, the polymer binder must be chemically compatible with both the electrolyte and the electrode materials. For alkali solutions, binders such as Teflon (PTFE), polycarbonate, PVC, polypropylene and rubber are preferred.
Other uses according to the present invention include lead acid battery where one or both electrodes are composite electrodes which comprise the proton conducting matrix of the invention. Also, the present invention encompasses lead acid batteries comprising PCMs of the invention.